© Semiconductor Components Industries, LLC, 2002

August, 2002 – Rev. 3

1

Publication Order Number:

AN1042/D

AN1042/D

High Fidelity Switching

Audio Amplifiers Using

TMOS Power MOSFETs

Prepared by: Donald E. Pauly

ON Semiconductor

Special Consultant

Almost all switching amplifiers operate by generating a

high frequency square wave of variable duty cycle. This

square wave can be generated much more efficiently than

an analog waveform. By varying the duty cycle from 0 to

100%, a net dc component is created that ranges between

the negative and positive supply voltages. A low pass filter

delivers this dc component to the speaker. The square wave

must be generated at a frequency well above the range of

hearing in order to be able to cover the full audio spectrum

from dc to 20 kHz. Figure 1 shows a square wave

generating a sine wave of one–ninth its frequency as its

duty cycle is varied.

0.5

1.0

0

°

90

°

180

°

270

°

360

°

420

°

0.75

0.25

–0.5

0

–0.25

–0.75

–1.0

Input

Output

Switching Frequency =

9X Modulation Frequency

0

+1

–1

Figure 1. Switching Amplifier Basic Waveforms

The concept of switching amplifiers has been around for

about 50 years but they were impractical before the advent

of complementary TMOS power MOSFETs. Vacuum tubes

were fast enough but they were rather poor switches. A

totem pole circuit with supply voltages of

±250 volts would

drop about 50 volts when switching a current of 200

milliamps. The efficiency of a tube switching amp could

therefore not exceed 80%. The transformer needed to

match the high plate impedance to the low impedance

speaker filter was impractical as well.

With the introduction of complementary bipolar power

transistors in the late 1960s, switching amplifiers became

theoretically practical. At low frequencies, bipolar transistors

have switching efficiencies of 99% and will directly drive

a low impedance speaker filter. The requirement for

switching frequencies above 100 kHz resulted in excessive

losses however. Bipolar drive circuitry was also complex

because of its large base current requirement.

With the advent of complementary (voltage/current

ratings) TMOS power MOSFETs, gate drive circuitry has

been simplified. These MOS devices are very efficient as

switches and they can operate at higher frequencies.

A block diagram of the amplifier is shown in Figure 2.

An output switch connects either +44 or –44 volts to the

input of the low pass filter. This switch operates at a carrier

frequency of 120 kHz. Its duty cycle can vary from 5% to

95% which allows the speaker voltage to reach 90% of

either the positive or negative supplies. The filter has a

response in the audio frequency range that is as flat as

possible, with high attenuation of the carrier frequency and

its harmonics. A 0.05 ohm current sense resistor (R27) is

used in the ground return of the filter and speaker to provide

short circuit protection.

The negative feedback loop is closed before the filter to

prevent instabilities. Feedback cannot be taken from the

speaker because of the phase shift of the output filter, which

varies from 0

° at dc to nearly 360° at 120 kHz. Since the

filter is linear, feedback may be taken from the filter input,

which has no phase shift. Unfortunately, this point is a high

frequency square wave which must be integrated to

determine its average voltage. The input is mixed with the

square wave output by resistors R4 and R5 shown in Figure

2. The resultant signal is integrated, which accurately

simulates the effect of the output filter. The output of the

integrator will be zero only if the filter input is an accurate

inverted reproduction of the amplifier input. If the output

is higher or lower than desired, the integrator will generate

a negative or positive error voltage. This error voltage is

applied to the input of the switch controller, which makes

the required correction. The integrator introduces a 90

°

phase shift at high frequencies which leaves a phase margin

of nearly 90

°.

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